Recently, tomography apparatuses have been widely used in order to obtain information of the inside of a living organism (subject). There are an X-ray computed tomography (hereinafter “X-ray CT”) apparatus, a magnetic resonance imaging (MRI) apparatus, a single photon emission CT (hereinafter “SPECT”) apparatus, and a positron emission tomography (hereinafter “PET”) apparatus, as the tomography apparatuses. In the X-ray CT apparatus, X-ray beams having narrow widths are emitted to a certain cross section of the subject in multiple directions, X-rays penetrating through the subject are detected, and a spatial distribution of the degree of abruption of the X-rays in the cross section is computed by a computer and imaged. Thus, dysplasia inside the subject such as a hemorrhagic area can be recognized by the X-ray CT.
On the other hand, since functional information in the subject can be obtained with high precision by the PET apparatus, development of the PET apparatuses has been progressing recently. In a diagnostic method using the PET apparatus, first, a medicine for inspection which is supplied with a positron nuclide is introduced inside the subject by an injection, inhalation, or the like. The medicine for inspection introduced in the subject is stored in a specific portion having a function corresponding to the medicine for inspection. For example, in a case where a medicine for inspection of saccharide is used, the medicine is selectively stored in a portion where metabolism of a cancer cell or the like frequently occurs. At this time, a positron radiates from the positron nuclide of the medicine for inspection. At the time when the positron and an electron in the periphery of the positron are coupled and annihilated, two gamma rays (so-called annihilation gamma rays) are radiated in directions approximately 180 degrees relative to each other. These two gamma rays are simultaneously detected by a radiation detector provided surrounding the subject and an image is regenerated by a computer or the like, so that image data of the distribution of the radioisotopes (RI) of the subject are obtained. Thus, in the PET apparatus, since the functional information about the body of the subject is obtained, it is possible to elucidate the pathology of various intractable diseases.
FIG. 1 is a schematic structural view of a PET apparatus. As shown in FIG. 1, in a PET apparatus 100, gamma ray detectors 101 are provided so as to surround a subject S 360 degrees. The gamma ray detector 101 includes a semiconductor detector 102 and a detection circuit 103. The semiconductor detector 102 includes a semiconductor detection device array (see FIG. 2) where semiconductor detection devices 104 (see FIG. 2) are arranged. The detection circuit 103 is configured to detect the gamma rays entering the semiconductor detection devices 104. In addition, a generating position of each gamma ray is identified based on an output signal from the detection circuit 103 indicating that the gamma ray has entered and position information of the semiconductor detection device 104 indicating where the gamma ray has entered. Furthermore, by detecting multiple gamma rays emitted in random directions, an image of the distribution of the medicine for inspection in the subject S is regenerated.
Thus, the X-ray CT apparatus or the PET apparatus configured to detect radiation such as the X-rays or the gamma rays so that pathological analysis is performed includes multiple semiconductor detection devices in order to improve detection efficiency, namely the amount or the number of radiations detected per unit time (for example, see Patent Document 1).    [Patent Document 1] Japanese Patent Application Publication No. 2001-242253